This won't help the OP other than to put the discomfort in perspective

The following is based on experience. It is clearly not suitable for any one allergic to stinging nettles. Arm yourself with a pair of thick gloves. Cut down a good quantity of fresh nettles. The best ones are young with pliable stems. A shopping bag full is about fine. Take a nettle and brush it against the head of your penis. If you haven't got an erection so far, this should bring one on. It doesn't hurt much, the sensation is rather good in fact. Brush some more nettles against your penis, the shaft as well. Gradually a rash will appear and little bumps. Keep going. The more you do, the greater the numbing effect so the next bit will be easier. Take a handful of nettles and crush them hard onto your penis. This may hurt, but rub them in well.
You can also take a nettle stalk (which is also covered in stingers) and wrap it around the shaft or behind the head. This is a good way of getting the poison in. Keep going with more nettles. As the poison gets in, the small bumps will join up. This is the desired effect.
As you keep on, the bumps turn into a large all over swelling. The more you do, the greater the swelling until your penis will be stretched real tight, bursting against its skin. I find an increase of about 50% in girth over a normal erection. The sensation is now between pain and pleasure. The nettles hurt, but the penis becomes super sensitive. As you go on the sensitivity will increase. Eventually you will reach the point where the sensitivity takes over from the pain.
Now your penis will start throbbing, but each throb will bring you closer and closer to orgasm. You can try to resist it, but it can be hard because chances are you will come without even touching your penis, the throbbing doing the job for you. The orgasm is intense - as much pain as pleasure. After orgasm you may remain quite erect, and a second spontaneous orgasm is possible within a very short time.
When finally flaccid, the penis will retain its girth but shorten, creating a massive, heavy weight swinging between your legs. The sensation will have gone, leaving your penis very numb to the touch.
A really weird feeling. Gradually - very gradually, the swelling will go down, but a huge lump may hang below the frenum for some time – a day or more. Now the skin has been so stretched it is very sore, and the sensation may make sleep difficult. Over the next few days the skin may start to peel off in a thin layer, leaving a new layer of soft skin underneath. Whether there is any permanent increase in size I couldn't say, but I guess the effect should be the same as a pump, if from the inside rather than the out.
Rubbing the stinging nettles into the scrotum and the anus can produce a good feel, but there is no swelling in the same way as the penis. You can also try filling your pants with nettles and taking a train ride. To do this, wear two pairs of pants, slip penis and scrotum through the fly of the first, and wrap over them a plastic bag full of nettles tied on loose with a rubber band. Try keeping a straight face as you walk, jog, cycle or ride a bumpy train.

Messiah - I shall be cycling sans shorts tonight and will report back on the levels of engorgement achieved!!

@cbmotorsport - the brambles round here in spring make my arms look like I self harm, they have had their fun by now and handed the baton on to the nettles who will then tag team the thistles after which it will be back to mud!